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Date:   Thu, 27 Dec 2018 16:54:48 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     alexei.starovoitov@...il.com, daniel@...earbox.net
Cc:     yhs@...com, netdev@...r.kernel.org, oss-drivers@...ronome.com,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Subject: [RFC bpf-next v2 03/12] bpf: verifier: remove dead code

Instead of overwriting dead code with jmp -1 instructions
remove it completely for root.  Adjust verifier state and
line info appropriately.

v2:
 - adjust func_info (Alexei);
 - make sure first instruction retains line info (Alexei).

Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
---
 include/linux/filter.h |   1 +
 kernel/bpf/core.c      |  12 +++
 kernel/bpf/verifier.c  | 197 ++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 207 insertions(+), 3 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 8c8544b375eb..2cdb50bbf867 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -782,6 +782,7 @@ static inline bool bpf_dump_raw_ok(void)
 
 struct bpf_prog *bpf_patch_insn_single(struct bpf_prog *prog, u32 off,
 				       const struct bpf_insn *patch, u32 len);
+int bpf_remove_insns(struct bpf_prog *prog, u32 off, u32 cnt);
 
 void bpf_clear_redirect_map(struct bpf_map *map);
 
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index a40057b2c556..cc6fa754627c 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -461,6 +461,18 @@ struct bpf_prog *bpf_patch_insn_single(struct bpf_prog *prog, u32 off,
 	return prog_adj;
 }
 
+int bpf_remove_insns(struct bpf_prog *prog, u32 off, u32 cnt)
+{
+	/* Branch offsets can't overflow when program is shrinking, no need
+	 * to call bpf_adj_branches(..., true) here
+	 */
+	memmove(prog->insnsi + off, prog->insnsi + off + cnt,
+		sizeof(struct bpf_insn) * (prog->len - off - cnt));
+	prog->len -= cnt;
+
+	return WARN_ON_ONCE(bpf_adj_branches(prog, off, off + cnt, off, false));
+}
+
 void bpf_prog_kallsyms_del_subprogs(struct bpf_prog *fp)
 {
 	int i;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 567ebe8baab4..e562b6112871 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6233,6 +6233,171 @@ static struct bpf_prog *bpf_patch_insn_data(struct bpf_verifier_env *env, u32 of
 	return new_prog;
 }
 
+static int adjust_subprog_starts_after_remove(struct bpf_verifier_env *env,
+					      u32 off, u32 cnt)
+{
+	int i, j;
+
+	/* find first prog starting at or after off (first to remove) */
+	for (i = 0; i < env->subprog_cnt; i++)
+		if (env->subprog_info[i].start >= off)
+			break;
+	/* find first prog starting at or after off + cnt (first to stay) */
+	for (j = i; j < env->subprog_cnt; j++)
+		if (env->subprog_info[j].start >= off + cnt)
+			break;
+	/* if j doesn't start exactly at off + cnt, we are just removing
+	 * the front of previous prog
+	 */
+	if (env->subprog_info[j].start != off + cnt)
+		j--;
+
+	if (j > i) {
+		struct bpf_prog_aux *aux = env->prog->aux;
+		int move;
+
+		/* move fake 'exit' subprog as well */
+		move = env->subprog_cnt + 1 - j;
+
+		memmove(env->subprog_info + i,
+			env->subprog_info + j,
+			sizeof(*env->subprog_info) * move);
+		env->subprog_cnt -= j - i;
+
+		/* remove func_info */
+		if (aux->func_info) {
+			move = aux->func_info_cnt - j;
+
+			memmove(aux->func_info + i,
+				aux->func_info + j,
+				sizeof(*aux->func_info) * move);
+			aux->func_info_cnt -= j - i;
+		}
+	} else {
+		/* convert i from "first prog to remove" to "first to adjust" */
+		if (env->subprog_info[i].start == off)
+			i++;
+	}
+
+	/* update fake 'exit' subprog as well */
+	for (; i <= env->subprog_cnt; i++)
+		env->subprog_info[i].start -= cnt;
+
+	return 0;
+}
+
+static int bpf_adj_linfo_after_remove(struct bpf_verifier_env *env, u32 off,
+				      u32 cnt)
+{
+	struct bpf_subprog_info *need_first_linfo;
+	struct bpf_prog *prog = env->prog;
+	u32 i, l_off, l_cnt, nr_linfo;
+	struct bpf_line_info *linfo;
+
+	nr_linfo = prog->aux->nr_linfo;
+	if (!nr_linfo || !cnt)
+		return 0;
+
+	linfo = prog->aux->linfo;
+
+	/* progs are already adjusted/removed, if program starts on off, it may
+	 * had its start cut off and its line info may need to be preserved
+	 */
+	for (i = 0; i < env->subprog_cnt; i++)
+		if (env->subprog_info[i].start >= off)
+			break;
+	if (i < env->subprog_cnt && env->subprog_info[i].start == off)
+		need_first_linfo = &env->subprog_info[i];
+	else
+		need_first_linfo = NULL;
+
+	/* find first line info to remove */
+	for (i = 0; i < nr_linfo; i++)
+		if (linfo[i].insn_off >= off)
+			break;
+
+	/* count lines to be removed */
+	l_off = i;
+	l_cnt = 0;
+	for (; i < nr_linfo; i++)
+		if (linfo[i].insn_off < off + cnt)
+			l_cnt++;
+		else
+			break;
+
+	/* either we didn't actually cut the start or we can just use line info
+	 * of first instruction if it exists
+	 */
+	if (i < nr_linfo && linfo[i].insn_off == off + cnt)
+		need_first_linfo = NULL;
+	if (need_first_linfo) {
+		if (WARN_ONCE(!l_cnt,
+			      "verifier bug - no linfo removed, yet its missing"))
+			return -EINVAL;
+		if (WARN_ONCE(need_first_linfo->linfo_idx < l_off ||
+			      need_first_linfo->linfo_idx >= l_off + l_cnt,
+			      "verifier bug - removed prog linfo not in removed range"))
+			return -EINVAL;
+		/* subprog linfo_idx is not adjusted yet, so just find out
+		 * which line it used to be and swap it
+		 */
+		memmove(linfo + l_off, linfo + need_first_linfo->linfo_idx,
+			sizeof(*linfo));
+		need_first_linfo->linfo_idx = l_off;
+		linfo[l_off].insn_off = off;
+
+		l_off++;
+		l_cnt--;
+	}
+
+	/* remove the line info which refers to the removed instructions */
+	if (l_cnt) {
+		memmove(linfo + l_off, linfo + i,
+			sizeof(*linfo) * (nr_linfo - i));
+
+		prog->aux->nr_linfo -= l_cnt;
+		nr_linfo = prog->aux->nr_linfo;
+	}
+
+	/* pull all linfo[i].insn_off >= off + cnt in by cnt */
+	for (i = l_off; i < nr_linfo; i++)
+		linfo[i].insn_off -= cnt;
+
+	/* fix up all subprogs (incl. 'exit') which start >= off */
+	for (i = 0; i <= env->subprog_cnt; i++)
+		if (env->subprog_info[i].linfo_idx >= l_off + l_cnt)
+			env->subprog_info[i].linfo_idx -= l_cnt;
+
+	return 0;
+}
+
+static int verifier_remove_insns(struct bpf_verifier_env *env, u32 off, u32 cnt)
+{
+	struct bpf_insn_aux_data *aux_data = env->insn_aux_data;
+	unsigned int orig_prog_len = env->prog->len;
+	int err;
+
+	if (!cnt)
+		return 0;
+
+	err = bpf_remove_insns(env->prog, off, cnt);
+	if (err)
+		return err;
+
+	err = adjust_subprog_starts_after_remove(env, off, cnt);
+	if (err)
+		return err;
+
+	err = bpf_adj_linfo_after_remove(env, off, cnt);
+	if (err)
+		return err;
+
+	memmove(aux_data + off,	aux_data + off + cnt,
+		sizeof(*aux_data) * (orig_prog_len - off - cnt));
+
+	return 0;
+}
+
 /* The verifier does more data flow analysis than llvm and will not
  * explore branches that are dead at run time. Malicious programs can
  * have dead code too. Therefore replace all dead at-run-time code
@@ -6293,6 +6458,30 @@ static void opt_hard_wire_dead_code_branches(struct bpf_verifier_env *env)
 	}
 }
 
+static int opt_remove_dead_code(struct bpf_verifier_env *env)
+{
+	struct bpf_insn_aux_data *aux_data = env->insn_aux_data;
+	int insn_cnt = env->prog->len;
+	int i, err;
+
+	for (i = 0; i < insn_cnt; i++) {
+		int j;
+
+		j = 0;
+		while (i + j < insn_cnt && !aux_data[i + j].seen)
+			j++;
+		if (!j)
+			continue;
+
+		err = verifier_remove_insns(env, i, j);
+		if (err)
+			return err;
+		insn_cnt = env->prog->len;
+	}
+
+	return 0;
+}
+
 /* convert load instructions that access fields of a context type into a
  * sequence of instructions that access fields of the underlying structure:
  *     struct __sk_buff    -> struct sk_buff
@@ -7032,11 +7221,13 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
 	if (is_priv) {
 		if (ret == 0)
 			opt_hard_wire_dead_code_branches(env);
+		if (ret == 0)
+			ret = opt_remove_dead_code(env);
+	} else {
+		if (ret == 0)
+			sanitize_dead_code(env);
 	}
 
-	if (ret == 0)
-		sanitize_dead_code(env);
-
 	if (ret == 0)
 		/* program is valid, convert *(u32*)(ctx + off) accesses */
 		ret = convert_ctx_accesses(env);
-- 
2.19.2

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